This invention relates to methods and apparatus for recovering high viscosity oils from subsurface earth formations, and more particularly relates to improved methods for recovering such oils by employing a large diameter shaft hole and a plurality of horizontal drill holes extending radially from the shaft hole.
Early disclosures relating to the recovery of petroleum substances by utilizing a large diameter shaft hole and a plurality of substantially horizontal drill holes are provided in U.S. Pat. Nos. 1,520,737 and 1,634,235, and a paper published by Ranney in the Petroleum Engineer in 1939 entitled "The World's First Horizontal Well". These publications propose the drilling of a large diameter shaft into an oil-bearing formation and then drilling radial drill holes into the formation. More recently, U.S. Pat. Nos. 4,020,901; 4,099,570; 4,099,783; 4,116,275; 4,160,481; and 4,201,420 provide improved systems for recovering petroleum substances employing large diameter shaft holes and radial drill holes.
Some of the above processes, however, suffer limitations relating to restrictions on the rate of introducing the injected fluid into the formation, which reduces the oil recovery rate. The techniques described in the above-cited patents may be suitable for recovering oil in some formations, but are not believed to be economically feasible for recovering oil from many formations. More particularly, such techniques are believed to recover a relatively low percentage of the oil in the formation, and many of these techniques demand high fuel requirements which further reduce the net oil recovery rate.
Oil recovery techniques utilizing conventional vertical wells can be generally classified as "drive" operations or "soak" operations. In a "drive" operation, a fluid is generally injected into the formation at a first location to form a wall for driving the oil in the formation toward recovery at a second location. The objective of a drive operation is to form a displacing fluid boundary and then drive the boundary through the formation utilizing the pressure of the injected fluid. Thus, in a dynamic drive operation, fluid is injected at a first point while oil is being recovered from a second point.
A major problem with most dynamic drive operations is that care must be taken to keep the injected fluid front in a wall configuration during the driving process. In operation, several factors naturally contradict the ideological driving wall of the dynamic drive operation, including variance in formation matrix, density and viscosity variations between the injected fluid and the oil in the formation, pressure and temperature changes about the formation, and gravitational forces.
When the driving wall breaks down, the phenomenon is typically referred to as either "fingering" or "gravity override", which are discussed in detail in the co-pending patent applications. A breakdown of the dynamic driving wall causes a significant reduction in the oil recovery efficiency of the dynamic driving process, and is a principal reason for its limited applicability.
In conventional vertical well "soak" operations, a solvent or steam may be injected into the formation for the purpose of reducing the viscosity of the oil, thereby allowing the oil to flow by gravity to recovery lines. "Soak" operations are generally not concerned with generating a wall of injected fluid, but are principally concerned with filling the formation with the soaking fluid to reduce the viscosity of the oil throughout the formation. In soak operations, the reduced viscosity oil generally flows by gravitational forces to recovery wells, and may be recovered at the same locations fluid is injected. Since a dynamic drive is not desired for a soaking process, a soaking fluid is generally not injected during the time interval in which oil is being recovered.
Soak operations are generally burdened, however, with relatively poor oil recovery efficiencies. Thus, it is not uncommon to soak a formation several times over a period of years. The cost of the fuel to generate steam is a major deterrent to the economic feasibility of soak operations. In addition, the slow rate of recovery common to soak operations substantially increases the cost of recovering oil, since expensive steam generating and water treatment equipment must be available during the life of the soak operation.
Conventional vertical well soak and drive techniques, when applied to horizontal well technology, are subjected to additional and/or different problems. One of the major problems with soak technology in conventional vertical wells is that the steam must invade or sweep through all the formation, and does not inadvertently rise over a portion of the formation because of the low density of steam. In horizontal wells, on the other hand, steam may be injected into the bottom of the wells along the length of the laterals, and thus the formation is more effectively saturated with steam since the steam rises naturally through the formation. Thus, horizontal well operations are not typically concerned with problems associated with soaking the entire formation, as in the case of conventional vertical well soak operations. Also, horizontal well soak operations are generally thought to be much more efficient than vertical well soak operations, since oil may be recovered over the long length of the laterals at the bottom of the formation, as compared to the points of recovery provided at the bottom of a formation by conventional vertical wells. Nevertheless, many horizontal well soak operations suffer from slow recovery rates and poor overall recovery efficiency.
Drive technology also becomes substantially altered when adapted to horizontal well configurations. Driving horizontally between horizontal wells commencing from a large diameter vertical well may not be practical because of difficulty in maintaining an effective wall of driving fluid. Also, since the spacing between adjacent laterals will vary with the distance from a common large diameter vertical well, the conventional univelocity wall driving techniques may not be applicable. Further, the close proximity of the laterals near a common diameter vertical well may result in steam fingering horizontally and short-circuiting between adjacent laterals.
The distinction between drive and soak operations is not always as simplistic as described above, although fundamental differences exist between these two techniques. Also, as previously described, a substantial variance exists between drive and soak operations adapted for horizontal well technology and drive and soak operations adapted for vertical well technology. Although horizontal wells generally offer the advantage of increased efficiency of oil recovery as compared with vertical wells, the costs associated with mining and operating a horizontal well are often heretofore prohibitive. Also, although horizontal steam soak operations are generally more efficient than vertical well steam soak operations, the efficiency of prior art horizontal well soak operations, when combined with the increased economic investment for horizontal wells, is such that oil recovery may not be practical.
The problems and disadvantages of the prior art are overcome with the present invention. Novel methods are herein provided for recovering high viscosity oils from a subsurface earth formation, wherein a greater percentage of the oil can be recovered from the formation, and can be recovered in a shorter time period.